Thin film heterojunction photovoltaic devices

ABSTRACT

Thin films of Hg 1-x  Cd x  Te with controlled x greater than 0.5 are cathodically deposited on a thin CdS film over a conductive film of ITO deposited on a glass substrate. Depositing a conductive film on the electrodeposited Hg 1-x  Cd x  Te treated to provide a Te-rich surface for a good ohmic contact forms an improved solar cell.

This is a division of application Ser. No. 576,559 filed Feb. 3, 1984,now U.S. Pat. No. 4,548,681.

BACKGROUND OF THE INVENTION

This invention relates to thin film photovoltaic devices that utilize Cdrich Hg_(1-x) Cd_(x) Te as a variable bandgap material, and to thecathodic electrodeposition of Hg_(1-x) Cd_(x) Te thin films withcontrolled stoichiometry (1-x) and thus with controlled electronic andoptical properties.

General electrodeposition procedures for CdTe have been given in U.S.Pat. No. 4,400,244 granted to F. A. Kroger, R. L. Rod and M. P. R.Panicker, and assigned to Monosolar, Inc. Briefly, to form a cadmiumtelluride coating on a conductive cathode, the electrolyte consists ofHTeO₂ ⁺ as the source of tellurium and Cd²⁺ as the source of cadmium.Discharged HTeO₂ ⁺ ions at the cathode reacts with Cd²⁺ and form CdTedeposit on the cathode.

More specific conditions for CdTe electrodeposition and details of aprocess utilized to make thin film heterojunction solar cells usingthese films have been described in U.S. Pat. No. 4,388,483 granted to B.M. Basol, E. S. Tseng and R. L. Rod and assigned to Monosolar, Inc.Briefly in this patent, a sheet of an insulating transparent material,such as glass, is prepared with, on one side, a transparent conductivefilm, such as a tin oxide or indium tin oxide (ITO) layer, usingconventional deposition techniques. Then a layer of a semiconductor,such as cadmium sulfide is electrodeposited. The combination of theconductive oxide and the cadmium sulfide comprise an n-type wide bandgapsemiconductor different from the next layer deposited, which is cadmiumtelluride. This structure is then heat treated at a temperature between250° and 500° C. for a time sufficient to convert the CdTe film to asubstantially low resistivity p-type semiconductor compound. Aconductive film, such as gold is then deposited on the cadmium tellurideto complete the photovoltaic cell which receives radiation through theglass substrate and the n-type semiconductor acting as a wide bandgapwindow.

Heat treating the cadmium telluride was found to increase the poweroutput of the photovoltaic cell by a factor of 60. It is believed that,in the absence of heat treatment, the electrodeposited cadmium tellurideis a high resistivity n-type material and the cadmium sulfide serves asan electron injecting contact to one surface of the CdTe film ratherthan a rectifying contact. When the top conductor (e.g., gold) isdeposited over the surface of the CdTe film, an n-CdTe/Au Schottkybarrier is obtained. This is intrinsically a low efficiency structure.When heat treated (before depositing the Au), substantially all of theCdTe is converted to p type, due apparently to the generation ofelectrically active Cd vacancies. This shifts the barrier from then-CdTe/Au interface to the CdS/p-CdTe interface and gives a highefficiency heterojunction structure.

Hg_(1-x) Cd_(x) Te is a very important infrared detector material. Itsbandgap is a function of its stoichiometry and can be changed from 0 to1.5 eV going from x=0.17 to x=1.0. So far the interest in this materialhas been limited to the infrared applications. Early work on Hg₀.795Cd₀.205 Te detectors (sensitive at λ=8-12 μm) was later followed byinvestigation of structures that are suitable for use in the 1-3, 3-5,and 15-30 μm range. All these applications require a Hg rich material(x<0.5). A survey of previous literature shows no successful attempt ofutilizing Cd rich (x>0.5) mercury cadmium telluride for solar cellapplications.

Hg_(1-x) Cd_(x) Te crystals can be prepared by techniques well known inthe art (such as Bridgman growth, zone melting, and Czochralski).Epitaxial growth can be achieved by (liquid phase epitaxy LPE) and(vapor phase epitaxy VPE). There has not been much work onpolycrystalline thin films of Hg_(1-x) Cd_(x) Te.

From this review of the prior art, it is apparent that there has been afailure to appreciate the potential of cadmium rich polycrystallineHg_(1-x) Cd_(x) Te for solar cell applications. This may partly be dueto the difficulties associated with the preparation of such films in aninexpensive way and with controlled stoichiometry.

Again the review of the prior art shows the lack of an inexpensivemethod for the production of Hg_(1-x) Cd_(x) Te films. The property ofbandgap control for Hg_(1-x) Cd_(x) Te is extremely important for highefficiency stacked cells where two or more cells respond to differentparts of the solar spectra. In the area of thin-film amorphous cells,there has been extensive research on variable bandgap alloys (such asamorphous Si-Ge alloys) that would be compatible with the top amorphousSi cell. But until this invention there has not been any success infinding a variable bandgap polycrystalline thin film that can becontrollably and inexpensively produced and utilized.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to demonstrate the utilization ofCd rich Hg_(1-x) Cd_(x) Te thin films in solar cells. As a result ofthis invention, cells sensitive to different portions of the solarspectra can be constructed and thus make possible the production ofstacked cells (tandem cells) with high efficiencies as well as thesingle junction cells with uniform or graded bandgaps.

Another object is to provide an inexpensive electrodeposition techniquethat is capable of yielding Hg_(1-x) Cd_(x) Te films with controlledelectronic and optical properties.

Yet another object of the present invention is to form heterojunctionthin film photovoltaic cells with electrodeposited Hg_(1-x) Cd_(x) Telayers.

Still another object of this invention is to teach a specificcomposition of the electroplating bath that yields high efficiencyHg_(1-x) Cd_(x) Te and CdTe solar cells.

In accordance with the present invention, thin films of polycrystallineHg_(1-x) Cd_(x) Te, with controlled Hg content are cathodicallyelectrodeposited on conductive substrates from an aqueous solutioncontaining 0.2M to 1.5M of Cd²⁺ ions, 10⁻⁵ M to 10⁻³ M of HTeO₂ ⁺ ionsand Hg²⁺ ions. The concentration of the Hg²⁺ ions is adjusted to 1 to 20ppm depending upon the desired stoichiometry of the deposit. The pH ofthe electrolyte is adjusted to a level between 1 and 3. The appliedpotential is adjusted so that the potential of the surface of thedeposit with respect to a Ag-AgCl reference electrode under open circuitcondition (i.e., QRP, Quasi Rest Potential) is between -300 mV and -600mV. The temperature of the electrolyte is kept around 85° to 90° C.Although the primary interest of the present electrodeposition processis cations (Cd²⁺ and HTeO₂ ⁺), the nature of the anions also affects thefilm properties. The addition of Cl-ions in the bath, for example,improves the short circuit current of photovoltaic cells, as will bedescribed in Example 3.

Thin film solar cells are produced by depostiting layers of Hg_(1-x)Cd_(x) Te on the CdS film of a glass/ITO/CdS substrate. The CdS film iselectrodeposited on the ITO coated glass using an electrolyte thatconsists of 0.1M to 0.5M cadmium sulfate or cadmium chloride and about0.01M to 0.05M of sodium thiosulfate with a pH of about 4 at thebeginning of the plating. The deposition voltage is kept between -0.6and -0.7 volts with respect to a calomel reference electrode, and thebath temperature about 90° C.

Device processing includes an annealing step (8-10 minutes at about 400°C.) which forms the rectifying junction at the CdS/Hg_(1-x) Cd_(x) Teinterface. After etching the surface of the Hg_(1-x) Cd_(x) Te film, theetched surface is treated with a strong basic solution. Devices arecompleted by depositing metal ohmic contacts on the etched and treatedsurface. Cells responsive to various wavelengths are produced bychanging the stoichiometry of the Hg_(1-x) Cd_(x) Te films, i.e., bychanging (1-x).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the chemical electrodeposition set-upfor the present invention.

FIG. 2 is a graph of the Hg²⁺ concentration in the solution with respectto the measured stoichiometry (1-x) of electrodeposited Hg_(1-x) Cd_(x)Te films, demonstrating the stoichiometry control possible with thepresent invention.

FIG. 3 is a graph of the energy gap (E_(g)) vs the Hg content inHg_(1-x) Cd_(x) Te films derived from the transmission/reflection dataof optical measurements.

FIG. 4 is a cross sectional view of a photovoltaic cell constructed inaccordance with the present invention.

FIG. 5 shows graphs of spectral responses of thin film Hg_(1-x) Cd_(x)Te solar cells which can be tailored by control of x in theelectrodeposition process.

FIG. 6 is a graph of V_(oc) and I_(sc) as a function of (1-x) in thinfilm photovoltaic devices utilizing thin film Hg_(1-x) Cd_(x) Te.

Various examples will now be given to show (1) how Hg_(1-x) Cd_(x) Tefilms can be electrodeposited, (2) how thin film solar cells can bemanufactured using these films and (3) how their performance can beimproved.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the drawings, a chemical electrodepositionset-up useful for the present invention is shown schematically. It isessentially the same as for any electrodeposition process in that itutilizes a vessel 10 to hold the electrolyte, which forelectrodepositing Hg_(1-x) Cd_(x) Te is an aqueous solution containingCd²⁺, HTeO₂ ⁺ and Hg²⁺ ions. The principal anode is a Te anode 12, andthe cathode is a conductive film 14 on a glass substrate 16. A referenceelectrode 18 is connected to a potentiostat/galvanostat 20. Athermometer 22 is used to monitor the bath temperature. To that generalset-up, there is added an inert graphite anode 24 and a switch 26 foralternately connecting the anodes to the potentiostat/galvanostat 20through which the power is applied in a controlled manner. The switch isshown as a manual switch, but in practice the switch is electronic andis controlled by a timing circuit 30.

EXAMPLE 1 (Electrodeposition of Hg_(1-x) Cd_(x) Te)

An electrolytic bath was prepared in accordance with the followingprocedures: ACS grade CdSO₄ was dissolved into double distilled water ina 3 liter beaker. The volume of the electrolyte was 1.6 liter and Cd²⁺concentration was 0.5M. The pH was 4.3. The beaker was placed on a hotplate and the solution was heated up to 90° C. while stirring it at thesame time with a magnetic stirrer. Then the electrolyte was purified fortwo hours using the inert graphite anode 24 and a platinum gauzecathode. The cathode potential was kept at -620 mV with respect to aAg-AgCl reference electrode 18 during this dummy plating which loweredthe impurity concentration in the bath to acceptable levels. Afterpurification, 0.015M of Cl⁻ was added into the solution using HCl. Thisis a crucial step for getting high efficiency devices. A separateexample set forth hereinafter will demonstrate this fact.

After adding Cl⁻, the pH was adjusted to 1.6 (at room temperature) byadding concentrated H₂ SO₄ into the solution. This was followed by theintroduction of HTeO₂ ⁺ into the electrolyte. HTeO₂₊ was introduced byusing the pure Te anode 12 and a Pt gauze cathode. The potential of theTe anode was kept at +500 mV with respect to the Ag-AgCl referenceelectrode 18 until the tellurium concentration (as monitored by anatomic absorption spectrophotometer not shown in FIG. 1) of 38 ppm wasreached. After plating nine CdTe films on glass/ITO/CdS substrates, 3.5ppm of Hg²⁺ was added into the electrolyte from a 1000 ppm of HgCl₂stock solution. Hg_(1-x) Cd_(x) Te film was deposited on 5×6 cm area ofa glass/ITO/CdS substrate using both anodes 12 and 24 (a Te rod and agraphite rod, respectively). The tellurium concentration in the solutionwas kept around 3×10⁻⁴ M by controlling the switching time of the timer30 which alternately switches between the two anodes. In the presentexample, the tellurium anode 12 was in the circuit for one minute andthe graphite anode 24 was in for 15 seconds alternately throughout thedeposition. The QRP (Quasi Rest Potential) was kept around -600 mV to-700 mV except during the first few minutes when it was lower. Table Ishows the plating parameters throughout deposition.

                  TABLE 1                                                         ______________________________________                                        Plating Time                                                                           -V.sub.applied (mV)                                                                         I (mA)  -QRP (mV)                                      ______________________________________                                        30   sec     865           10    350                                          1    min     890           9.6   540                                          2    min     895           9.9   680                                          3    min     895           9.7   690                                          5    min     888           9.6   680                                          14   min     885           9.6   675                                          35   min     885           10.2  675                                          52   min     885           9.9   655                                          1 hr 10 min                                                                            885           10.3    655                                            2    hrs     885           11.0  625                                          ______________________________________                                    

The resulting film (No. 1) was dissolved in HNO₃ and its chemicalcomposition was measured by atomic absorption spectrophotometer. It wasfound to contain Cd, Te and Hg but the question of whether the depositwas in the form of a compound (Hg_(1-x) Cd_(x) Te) or a mixture could beanswered only by optical measurements. When the optical bandgap of thefilm was measured, it was found to be smaller than that of CdTe therebyproving the existence of the Hg_(1-x) Cd_(x) Te compound.

EXAMPLE 2 (Stoichiometry Control of Hg_(1-x) Cd_(x) Te)

To demonstrate the stoichiometry control made possible by the presenttechnique, two more films (films No. 2 and No. 3) were deposited usingthe solution prepared for Example 1 and adding more Hg into thissolution. The Hg²⁺ concentration in the solution with respect to themeasured stoichiometry (1-x) of the resulting Hg_(1-x) Cd_(x) Te filmsis indicated in FIG. 2. Tables 2 and 3 show the plating parameters forfilms No. 2 and No. 3 respectively. Again, it should be noted that theQRP is low at the beginning of the plating and then it goes up,stabilizing at a level more positive than -700 mV.

                  TABLE 2                                                         ______________________________________                                        Plating Time                                                                           -V.sub.applied (mV)                                                                         I (mA)  QRP (mV)                                       ______________________________________                                        30   sec     860           9.9   350                                          1.5  min     885           10.0  350                                          2.5  min     900           9.8   400                                          3    min     910           10.1  655                                          6    min     907           10.5  785                                          7    min     890           9.6   740                                          8    min     875           9.3   680                                          10   min     875           9.2   625                                          12   min     880           9.3   600                                          15   min     885           9.2   650                                          22   min     885           9.6   650                                          45   min     885           9.8   645                                          1    hr      885           9.5   635                                          11/4 hr      885           9.3   655                                          13/4 hr      885           9.5   625                                          2    hrs     885           9.5   615                                          ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Plating Time                                                                           -V.sub.applied (mV)                                                                         I (mA)  QRP (mV)                                       ______________________________________                                        30   sec     860           9.1   350                                          1    min     880           9.7   400                                          1.5  min     905           9.8   755                                          2.5  min     885           9.4   685                                          4.5  min     885           8.9   600                                          7    min     890           8.7   660                                          10   min     890           9.1   700                                          20   min     885           9.2   670                                          35   min     885           9.3   680                                          1    hr      885           9.4   680                                          1 1/6                                                                              hr      885           9.4   680                                          1 hr 58 min                                                                            885           9.7     650                                            2    hrs     885           9.8   680                                          ______________________________________                                    

FIG. 2 shows that a controlled change in stoichiometry is possible bycontrol of the mercury content in the solution. The effect of thischange in stoichiometry on the electrical and optical properties of thefilm was studied by optical measurements and also by making solar cellsusing these films. The following section describes the production ofsuch devices. The energy gap (E_(g)) vs the Hg content in the films wasderived from the transmission/reflection data of optical measurementsand plotted in FIG. 3. It is observed that the bandgap values follow thetheoretically expected linear dependence on (1-x). This demonstrates thestoichiometry control of bandgap for photovoltaic devices made possiblewith the simple, low-cost deposition technique of the present invention.

PRODUCTION OF DEVICES FROM FILMS OF EXAMPLES 1 and 2

The films of Examples 1 and 2 were further processed to make thin filmphotovoltaic cells for the purpose of demonstrating the possibility ofproducing bandgap tailored, low-cost, thin-film devices. The deviceillustrated schematically in FIG. 4 is comprised of a sheet 40 ofinsulating transparent material (glass) having a layer 41 of conductivetransparent material (ITO) on which a film 42 of a semiconductor (CdS)was electrodeposited before electrodepositing film 43 of Hg_(1-x) Cd_(x)Te. A thin film 44 of conductive material (Au or Ni) was then evaporatedon the film 43 for use as the back contact. A front contact was made tothe conductive film 41 by first etching away the semiconductor layers 42and 43, thus exposing the conductive film 41 in an area to one side ofthe device.

If the Hg_(1-x) Cd_(x) Te films of Examples 1 and 2 were not heattreated before the deposition of the thin film 44 Schottky barrier solarcells responding to different wavelengths were obtained.

The heterojunctions were produced if the films of Examples 1 and 2 werefirst heat treated in accordance with the aforesaid U.S. Pat. No.4,388,483. Heat treatment was carried out at 400° C. in air for 8minutes. This step is believed to generate Cd and Hg vacancies in thefilms which act as acceptors and give rise to a suitably low resistivityp-type material. After the heat treatment films were cooled down to roomtemperature, the following etching and relating procedures wereperformed:

a. The surface of the Hg_(1-x) Cd_(x) Te film 43 was first etched in a0.1% bromine in methanol solution. This etch removes a very thin layerof material (<100 Å) and leaves a clear working surface. This is not acrucial step in the process, it can be left out if the film 43 isfreshly made.

b. Then the surface was etched in a dichromate solution ("Dichrol" byAmerican Scientific Products) for one second and rinsed under D.I.water. This etch leaves a Te-rich surface which is necessary for a goodohmic contact.

c. After the dichromate etch, the sample was immersed into a beakerfilled with hydrazine (monohydrate by Fisher Scientific Company) for tenminutes at room temperature. As described in U.S. Pat. No. 4,456,630,this step, along with step b, is important in device processing. Ittreats the surface of the film in a way to eliminate any high resistanceor barrier associated with the ohmic contact.

d. Following the surface preparation steps described above, a metal film44, such as Au or Ni, was evaporated onto the surface treatedsemiconductor film. This metal film constituted the back contact to thefinished cell.

e. A front contact was then made by removing the films 44 43, and 42, asnoted above, to expose the conductive film 41 of ITO.

The devices were then checked for their spectral responses and V_(oc),I_(Sc) values.

FIG. 5 illustrates how the response of a thin film solar cell can betailored utilizing the present invention. Curves a, b, c and d in FIG. 5correspond to (1-x)=0, 0.075, 0.105 and 0.125, respectively. Theextension of the cell response deeper into the infrared region withlarger (1-x) values is clear from this figure. FIG. 6 shows the V_(oc)and I_(sc) values of cells (0.02 cm² area) made on films that werepreviously used in the badgap measurements (FIG. 3). As expected fromthe change in the bandgap, the open circuit voltage decreases and theshort circuit current increases with increased Hg content in the films.

EXAMPLE 3

This example demonstrates the importance of C1⁻ addition into theelectrolyte. The following experiment was carried out to study thisphenomenon:

A CdTe plating bath was prepared in accordance with the followingprocedures:

a. 700 ml, 0.5M, ACS grade CdSO₄ solution was prepared using doubledistilled water. The solution was heated up to 90° C. and gentlystirred.

b. The electrolyte was purified for 2 hours using a platinum gauze as acathode, a graphite rod as the anode and a Ag-AgCl electrode as thereference. A PAR 173 potentiometer was used to apply a cathode potentialof -620 mV during this process.

c. The pH of the solution was adjusted to 1.6 by adding concentrated H₂SO₄.

d. Te was introduced into the electrolyte by applying +500 mV to a pureTe block anode and using Pt gauze as the cathode electrode. The Teintroduction was terminated when Te concentration reached 35 ppm.

After the preparation of this standard bath, the Cl⁻ concentration inthe solution was changed using concentrated HCl and the resulting filmswere used to make heterojunction solar cells following the proceduresdescribed in Example 3. Both the mechanical integrity of the films afterthe heat treatment step and the short circuit current densities of thedevices were evaluated as a function of Cl⁻ concentration. Table 4 showsthe results for 0.02 cm² area devices.

                  TABLE 4                                                         ______________________________________                                        Sample No.                                                                              [SO.sub.4.sup.-2 ]                                                                         [Cl.sup.-]                                                                             I.sub.sc (μA)                              ______________________________________                                        F22-1     0.5 M        0        225                                           F22-2     0.5 M        0        225                                           F22-3     0.5 M        0.0025 M 290                                           F22-4     0.5 M        0.005 M  330                                           F22-5     0.5 M        0.005 M  330                                           F22-6     0.5 M        0.005 M  350                                           F22-7     0.5 M        0.005 M  365                                           F22-8     0.5 M        0.01 M   380                                           F22-9     0.5 M        0.015 M  380                                           ______________________________________                                    

Although cells made with Cl⁻ concentrations of up to 0.03M in thesolution were still satisfactory, one could observe that the CdTe filmwas getting detached from the glass substrate at certain portions afterthe 400° C. heat treatment. So the preferred molar ratios of Cl⁻ ion toSO₄ ⁻² ion in the solution are 0.01 to 0.06. Ratios smaller than 0.01are not very effective and ratios greater than 0.06 give rise to pooradhesion between the substrate and the CdTe film. It is not yet veryclear how the presence of Cl⁻ in the electrolyte affects the propertiesof the deposited films. It may be that Cl⁻ has a compensating effect onthe grain boundaries which reduce the recombination velocity at thesesites or Cl⁻ may actually affect the growth morphology giving rise to astructure (such as good columnar growth) that yields higher shortcircuit current values in devices. All the above arguments areapplicable to Hg_(1-x) Cd_(x) Te deposition as well as to CdTedeposition. Naturally others of the more common halogen ions, namelyBr⁻, F⁻ and I⁻, can be used instead of Cl⁻.

From the foregoing, it is evident that a new and improved method ofelectrodepositing a film of Hg_(1-x) Cd_(x) Te has been disclosed, thatthe film may be cadmium rich, and the film may be provided with improvedshort circuit current in a photovoltaic cell by the inclusion of halideions. Consequently, it is also evident that the present inventionprovides a new and improved thin-film solar cell. In the examples given,reference has been made to potentials in the electrodeposition processwith respect to standard Ag-AgCl and calomel reference electrodes. Thesecould have been given with respect to a normal hydrogen electrode as thereference which is 0.22 V below the Ag-AgCl reference electrode and 0.24V below the calomel reference electrode. Since NHE is a standard at 0volts, the potentials in the claims that follow are with respect to thenormal hydrogen electrode. One can then choose which reference electrodeto use.

What is claimed is:
 1. A solar cell comprising at least one layer ofpolycrystalline cadmium-rich Hg_(1-x) Cd_(x) Te (x>0.5) containinghalide atoms, as an active solar energy absorbing layer.
 2. The solarcell of claim 1 wherein one surface of said at least one layer ofcadmium-rich Hg_(1-x) Cd_(x) Te is in ohmic contact with a conductivemetal layer; and the opposing surface of said at least one layer ofcadmium-rich Hg_(1-x) Cd_(x) Te contacts a layer of semiconductormaterial and forms a heterojunction with it.
 3. The solar cell of claim1 wherein one surface of said at least one layer of cadmium-richHg_(1-x) Cd_(x) Te forms a Schottky barrier with a conductive metallayer; and the opposing surface of said at least one layer ofcadmium-rich Hg_(1-x) Cd_(x) Te contacts a layer of a semiconductormaterial.
 4. The solar cell of claim 1 comprising a glass substratecoated with a conductive oxide layer, a layer of CdS disposed on saidconductive oxide, said cadmium-rich layer of Hg_(1-x) Cd_(x) Te disposedon said CdS layer, and a conductive metal layer disposed on and in ohmiccontact with said Hg_(1-x) Cd_(x) Te layer.
 5. The solar cell of claim 4having a tellurium-rich surface of said cadmium rich Hg_(1-x) Cd_(x) Telayer disposed adjacent the ohmic contact.